In a construction machine such as a hydraulic excavator, generally speaking, hydraulic oil is supplied to various hydraulic actuators from a hydraulic pump driven by an engine. A variable displacement pump, such as a swash plate pump or a bent axis pump, is used as the hydraulic pump. By changing the tilting angle of the hydraulic pump, the flow rate of the hydraulic oil discharged from the hydraulic pump is changed.
In general, the tilting angle of the hydraulic pump is adjusted by a regulator. For example, Patent Literature 1 discloses a hydraulic drive system that includes: two hydraulic pumps driven by one engine; and two regulators that adjust tilting angles of the respective hydraulic pumps. In the hydraulic drive system, in order to prevent an engine stall due to overload, horsepower control is performed so that the total horsepower of the hydraulic pumps will not exceed the engine output.
Specifically, in Patent Literature 1, discharge pressures of the two respective hydraulic pumps are led to each regulator. One of the discharge pressures is the discharge pressure of a regulator-side hydraulic pump coupled to the regulator, and the other discharge pressure is the discharge pressure of a counterpart hydraulic pump coupled to the other regulator. Each regulator increases the tilting angle of the regulator-side hydraulic pump in accordance with an increase in the discharge pressure of the regulator-side hydraulic pump and an increase in the discharge pressure of the counterpart hydraulic pump, thereby increasing the discharge flow rate of the regulator-side hydraulic pump. That is, the tilting angles of the two hydraulic pumps are always kept equal to each other. A control pressure from a proportional valve is also led to both the regulators, and the tilting angles of both the hydraulic pumps are increased in accordance with an increase in the control pressure. It should be noted that, in this technical field, horsepower control based on the discharge pressures of the regulator-side hydraulic pump and the counterpart hydraulic pump is often called total horsepower control, and horsepower control based on the control pressure is often called power shift control.
To be more specific, each regulator includes: a servo cylinder coupled to the regulator-side hydraulic pump; a spool for controlling the servo cylinder; and a horsepower control piston that pushes the spool in such a direction as to increase the discharge flow rate of the regulator-side hydraulic pump in accordance with an increase in the discharge pressure of the regulator-side hydraulic pump, an increase in the discharge pressure of the counterpart hydraulic pump, and an increase in the control pressure.
The hydraulic drive system disclosed in Patent Literature 1 is intended for a hydraulic excavator, and one of the hydraulic pumps supplies the hydraulic oil to, for example, a turning hydraulic motor via a control valve, and the other hydraulic pump supplies the hydraulic oil to, for example, a bucket cylinder via a control valve.